Pros: sound quality, comfortable to wear
Cons: long charging time, signal not strong
I can't believe there is no review on this headphone. I will write a brief one because I am really not equipped and skilled to write reviews on this forum as deep and broad as other reviews resulted from extensive tests and auditions.
For sound, I could only compare this one with its wired cousin HD650. I connected them to the headphone jack on an old receiver. I listened to various or materials, classical music, pop songs, R&R, vocal and instruments. The sound is very clear and sound stage is decent. treble, mid range and bass are all nicely rendered. I haven't used it on a movie. It cannot take in Dolby or DTS audio input directly and have to connect it to a receiver. I can't tell much difference between HD650 and RS220. I am not saying it's as good as HD650 but just that's it's very good.
They have optical, coaxial and analog input connections. Switching between them is just finger tap on the transmitter which is also the charging station.
Charging seems to be long because I saw the red charging indicator on for long time, more than 12 hours after use.
Now let's speak about the wireless parts. If it's charged well and I am in the same room, the signal transmission is solid and I do not suffer from signal drop or sound distortion. I can go to different floor in the room and listen. However, if I was blocked by two walls, I may experience signal drops and the sound may be distorted occasionally. Maybe their DSSS technologies haven't matured enough. I used my Bluetooth headphone Voxoa VHX330 in same spots and never lost signal. There are reviews of RS220 on other sites (mainly online stores) and majority of negatives are about the weak signals and Sennheiser's not satisfying response on this complain. I think there is room to improve on this part. But if you only use it in the same room or have no more than one wall blockage, you can be safe to use it.
The controls on the phone is awkward to use. The balance and volume are using the same button on the right phone. It may drive you nuts to use it because you do not know which function you are using. On the left phone the on/off and switching between inputs also share the same button.
I can listen for more than 2 hours without feeling any discomfort on the head or ears. It feels great and very comfortable.
Overall, it's a decent headphone and specifically wireless headphone. Some people like to compare it with RS 180 and I had no experience with that one. Wireless headphones use a few different technologies to transmit signals and this is probably the only one to use DSSS. For price at $500 it's not a good value. This price can be justified if the DSSS signal is solid and the phone also have wired connection option. But it's often sold at discount up to $200 and $300-400 is fair price. I think 4 stars ratings are affair for its performance and value if you get a good deal.
If you have Sennheiser HDR 220 you can use that one which is exactly the headphone for RS220. Yes, the transmitter supports multiple headphones (4 may be the limit).
Hope someone write reviews in much details.